The present invention relates to the technical field of nitration of toluene for preparation of dinitrotoluenes with subsequent removal of the impurities from the crude dinitrotoluenes (crude DNTs) for the purposes of providing pure dinitrotoluenes.
More particularly, the present invention relates to a process for purifying crude dinitrotoluenes that result from the nitration (dinitration) (e.g. adiabatic or isothermal nitration) of toluene in the presence of a nitric acid/sulfuric acid nitrating acid mixture (i.e., in other words, a process for preparing or providing pure or purified dinitrotoluenes), and a corresponding apparatus or plant for performing this process.
In addition, the present invention relates to a production plant for preparation of dinitrotoluenes (i.e. a production plant for nitration of toluene to dinitrotoluenes) with subsequent purification of the nitrated crude dinitrotoluenes formed in the nitration (crude DNTs).
Dinitrotoluenes (DNTs) are an important intermediate in the chemical industry, which, after reduction to the corresponding amines (for example by reaction with hydrogen in the presence of a catalyst), serve particularly as starting materials or reactants for polyurethanes.
In general, DNT is prepared as an isomer mixture by reacting toluene with nitric acid directly or in the presence of sulfuric acid as catalyst and water-binding agent in one stage or in two stages in countercurrent, isothermally or else adiabatically. After removal of the spent nitrating acid (i.e. generally a spent nitric acid/sulfuric acid nitrating acid mixture), a still variously contaminated crude dinitrotoluene is obtained, which is in a purity that allows further use or further processing only after passing through further, usually complex, treatment steps.
In the context of the present application, in relation to the dinitrotoluene(s) (DNT or DNTs), the singular or plural form is used synonymously hereinafter.
In the conversion of toluene to dinitrotoluene (DNT) with nitric acid, as well as the desired nitro species, a multitude of impurities, especially a multitude of very different by-products and other impurities, are present in the crude DNT, especially including traces of MNT (mononitrotoluene), TNT (trinitrotoluene) and residues of aliphatic and cycloaliphatic hydrocarbons that are always present in the starting toluene, and additionally a series of further by-products, for example mononitrocresols (MNK), dinitrocresols (DNK) and trinitrocresols (TNK), trinitrophenol (picric acid or PS), nitrobenzoic acids (NBS) such as mononitro-benzoic acid (MNBS) and dinitrobenzoic acid (DNBS), and degradation products from the oxidation of the aliphatic and cycloaliphatic hydrocarbons, and of the nitrocresols and nitroaromatics with the nitric acid from the mixed acid, for example carbon dioxide (CO2), carbon monoxide (CO), hydrogen cyanide (HCN), tetranitromethane (TNM), formic acid, acetic acid, oxalic acid, etc., and reaction or reduction products of nitric acid, such as nitrogen oxides (e.g. nitrogen oxide NO, nitrogen dioxide NO2, dinitrogen oxide N2O, etc.), nitrous acid, etc. In addition, nitric acid and sulfuric acid are usually present in the crude DNT, especially in dissolved form and/or as a microemulsion of the spent nitrating acid in finely divided form.
In general, all these impurities or at least a significant portion thereof have to be removed from the crude DNT prior to the further processing or further use thereof. The extent of the contamination, i.e. the residual content of the impurities in the purified nitroaromatic, is determined especially by the levels of purity required based on the use, which depend upon factors including the process envisaged for the further use or the like. In particular, all substances that disrupt further processing, even in traces, have to be reduced to a minimum, for example the hydrogenation-preventing substances, for example sulfur compounds, nitrogen oxides, nitric acid etc. (cf., for example, U.S. Pat. No. 4,224,249 A), nitrocresols (cf., for example, U.S. Pat. No. 2,976,320 A), carbon dioxide (cf. EP 1 935 870 A1), sodium ions or the known complexing agents and catalyst poisons, for example CO, N2O, hydrogen cyanide, etc.
According to the prior art, these impurities are typically removed from the crude nitroaromatics (crude DNTs) after separation (removal) of the spent nitrating acid by a wash in a plurality of wash steps before the nitroaromatic is sent to a direct use, an isomer separation or a hydrogenation to the corresponding amines.
Typically, the wash of the crude nitroaromatics for the removal of the acids which are dissolved and suspended in the nitration mixture, of the nitrophenols and of other acidic and other impurities that are still extractable with the washing agent consists of three steps (see, for example, F. Meissner et al., Industrial and Engineering Chemistry Vol. 46, 721 (1954); Ullmanns Enzyklopädie der technischen Chemie [Ullmann's Encyclopedia of Industrial Chemistry], 4th edition, vol. 17, pages 385/386; H. Hermann et al., ACS Symposium Series 632, 238 (1996), page 241 [editors: L. F. Albright, R. V. C. Carr, R. J. Schmitt]; A. B. Quakenbush et al., The Olin Dinitrotoluene (DNT) Process, Polyurethanes World Congress 1993, Publish: Technomic Lancaster, pages 484-488; U.S. Pat. No. 6,288,289 B1; EP 1 816 117 A1).
This three-stage wash of the crude nitroaromatics generally comprises the following wash steps:
1. Wash Step I or Acidic Wash:
                Wash with water (acidified as a result of the spent nitrating acid mixture), especially for removal of the dissolved and suspended strong mineral acids, such as sulfuric acid and nitric acid, and the nitroses (i.e. nitrogen oxides).2. Wash Step II or Alkali Wash (Basic Wash):        Wash in the presence of a base, for example sodium carbonate (soda), sodium bicarbonate, sodium sulfite or sodium hydrogensulfite, ammonia, sodium hydroxide solution, potassium hydroxide solution etc. (cf., for example, U.S. Pat. No. 4,482,769 A, U.S. Pat. No. 4,597,875 A or U.S. Pat. No. 6,288,289 B1), especially for removal of traces of nitric acid, nitrogen oxides and sulfuric acid that are still present in dissolved and suspended form in the nitroaromatics after wash step I, and of weakly acidic impurities still dissolved in the nitroaromatics, such as nitrophenols, nitrocresols, nitrobenzoic acids, degradation products from the oxidative breakdown of phenols or aliphatic and/or cyclic hydrocarbons, etc., for example hydrogen cyanide, oxalic acid, formic acid, acetic acid, etc.3. Wash Step III or Neutral Wash:        Neutral wash, especially for removal of the residual traces of alkalis or basic compounds from microemulsion or other entrainment from wash step II, and for further reduction of the impurities still present in traces in the product.        
It is possible in principle for every wash step to comprise up to 10 extraction stages in cross- or countercurrent or a combination thereof, as described, for example, in CA 1 034 603, in EP 1 780 195 A1, in U.S. Pat. No. 6,288,289 B1 or in WO 2013/160367 A1.
The wash medium used for the wash is typically water. In general, the wash is conducted as a liquid/liquid extraction, i.e. at temperatures where the nitroaromatic to be washed is in liquid form.
The aim of this generally multistage wash is to obtain, apart from a pure product which can be processed further without any adverse effect resulting from impurities still present, a minimum amount of wastewater per metric ton of product, by washing out the impurities that are present in such a manner that the disposal thereof can be conducted inexpensively, advantageously with recovery or recycling of unconverted reactants.
In the washing of the crude DNTs with water in the context of a liquid/liquid extraction, however, impurities washed out are predominantly only those which have a high solubility in the wash medium (for example sulfuric acid and nitric acid) or which disproportionate in the aqueous medium with the water to give nitric acid or which form salts with the bases added within the alkaline or basic wash (see wash step II above) and hence are removed irreversibly from their partition equilibrium with the DNT organic phase (for example residual traces of the strong acids [sulfuric acid and nitric acid] and the medium-strength and weak acids [for example nitrophenols, nitrocresols, nitrobenzoic acids, acetic acid, oxalic acid, formic acid, carbonic acid, hydrogen cyanide, etc.]). But even in relation to these impurities, the removal thereof in the wash is not always complete or quantitative, especially not when the crude DNTs have high loadings of impurities.
It has also been found, moreover, that other impurities, especially impurities having unfavorable partition coefficients in relation to the wash medium, impurities with weak or zero acidity and impurities having a slow conversion rate to such substances that do not dissolve in the organic phase, often cannot be removed completely with a wash having one or more steps in the context of a liquid/liquid extraction and/or can be removed only with a high level of technical complexity. This is true particularly when, for reasons of process economy (especially for reduction of the wastewater burden), the wash in the individual wash steps is effected with a ratio of wash medium used to washing nitroaromatic of less than 1 (for example at a volume ratio in the range from 1:5 to 1:20), even when the ratio of wash medium to nitroaromatic in the wash apparatus, through circulation of the wash medium, is 1 or even greater than 1. Such impurities that can be removed only with difficulty or only incompletely in the context of a wash typically include, but are not limited to, nitrogen oxides NOX (e.g. nitrogen oxide NO, nitrogen dioxide NO2, dinitrogen oxide N2O etc.), hydrogen cyanide, aliphatic and aromatic hydrocarbons, carbon oxides (CO, CO2 etc.) etc.
More particularly, it is found to be difficult to remove the impurities from the crude DNTs when only one wash step is used, for example an acidic wash only. As described in CA 1 034 603, U.S. Pat. No. 4,224,249 A, EP 0 297 312 A1, EP 0 736 514 A1 or EP 1 780 195 A1, the washed DNT still contains, as well as traces of the strong acids sulfuric acid, nitric acid and nitrogen oxides NOX, all medium-strength and weak organic and inorganic acids having pKA values equal to or greater than the pH of the wash acid, for example nitrocresols, nitrobenzoic acids, hydrogen cyanide, CO2, etc., and additionally also all neutral or nonacidic impurities, for example CO, N2O, hydrocarbons, etc., which, according to their solubility in the wash acid and depending on their individual partition equilibrium, are only partly washed out and hence remain in the DNT. It is particularly difficult here to remove the nitric acid, since NOX which is constantly dissolved in small amounts in the DNT, in the presence of traces of dispersed water, constantly reforms nitric acid.
According to CA 1 034 603, seven washes with water in crosscurrent (example 2) are required in order to attain a residual content of mineral acids in the DNT of about 15 ppm. The medium-strength and weak inorganic and organic acids remain in the organic phase at pH values less than 4 (after 7 water washes) and are extracted into the wash medium only in accordance with their partition equilibria.
According to U.S. Pat. No. 4,224,249 A, a DNT washed only with water contains less than 6000 ppm of total acidity and preferably less than 3000 ppm (calculated as sulfuric acid) of acids. Of this, 1100 ppm is nitrate, 160 ppm is nitrite and 230 ppm is sulfate. All weak acids, such as nitrocresols, and all neutral impurities, by contrast, remain in the DNT.
EP 0 297 312 A1, EP 0 736 514 A1 and EP 1 780 195 A1 disclose processes which permit conduction specifically of the wash step of the acidic wash so as to obtain a wash acid with maximum concentration of acid (a mixture of sulfuric acid, nitric acid and nitrous acid or NOX). This wash acid is recycled into the nitration directly or preferably after concentration to a total acid (calculated as nitric acid) of 50% to 60%. In order to achieve this enrichment of the sulfuric and nitric acid in the wash acid, the ratio of DNT to fresh water must be very high. Thus, in the working example of EP 0 736 514 A1, one metric ton of crude DNT is washed in multiple stages in countercurrent with about 110 liters of fresh water (corresponding to a ratio of DNT to fresh water of 9.1:1), in order to obtain a wash acid with about 23.3% total acid (corresponding to an amount of wash acid of about 142 kg per metric ton of DNT). With a ratio of DNT to fresh water for the acidic wash of about 13:1, only about 77 liters of fresh water would be required per metric ton of crude DNT to be washed, and a wash acid with more than 30% total acid would be obtained.
Given such a phase ratio of DNT to water of about 9:1 or 13:1, with impurities having extraction factors ε of 1 or less and partition coefficients in favor of the extractant of 10 or less, exhaustive extraction with extraction yields of 90% or more is enabled only as a result of a very high to theoretically infinite number of extraction stages (in this regard, cf., for example, K. Sattler, Thermische Trennverfahren [Thermal Separation Processes], Wiley-VCH publishers, 3rd edition 2001, pages 545 ff.); this is extremely technically demanding and very costly and, moreover, is not technically possible in the case of impurities having partition coefficients less than 5.
By contrast, by such a conventional one-stage or multistage wash in the form of a liquid/liquid extraction as described, for example, in the prior art according to EP 0 297 312 A1, EP 0 736 514 A1 and EP 1 780 195 A1, it is not possible to remove impurities having unfavorable partition coefficients in relation to the wash medium, impurities having low solubility in the wash medium, impurities having weak or zero acidity and impurities having a slow conversion rate to substances that do not dissolve in the organic phase from the crude DNT, or not possible to do so completely and efficiently. The impurities that cannot be removed in this way include, for example (but without restriction), nitrogen oxides NOX, carbon oxides such as CO and CO2, hydrogen cyanide, hydrocarbons, etc.
Even by a wash of crude DNTs in three wash steps each having up to 5 extraction stages according to the prior art, wherein a wash step is conducted in the presence of a base, preferably sodium carbonate, at a pH in the range from 8.5 to 9.2, only weak mineral acids such as carbonic acid, and nitrophenols and nitrocresols, and organic acids such as nitrobenzoic acids, acetic acid, oxalic acid, formic acid etc., having pKA values below 7 are washed out more or less quantitatively. But even hydrogen cyanide with a pKA of about 9.2, in the case of the standard wash with sodium carbonate in the pH range of 8.5 to 9.2, is extracted only partly from the crude DNT; the aromatic and aliphatic hydrocarbons and CO, N2O etc. can be removed by extraction only according to their solubilities and partition equilibria from the crude DNT. The same applies when ammonia in excess is used as base; traces thereof may then additionally be present in the washed DNT.
A wash to improve the extraction level of impurities with unfavorable partition coefficients and/or with low solubility in the wash medium or of impurities with weak or zero acidity (such as CO2, hydrogen cyanide, CO, N2O etc.) or of volatile or low-boiling aliphatic and aromatic hydrocarbons (such as hexane, pentane, cyclohexane, methylcyclohexane, dimethylcyclohexane, toluene etc.) or impurities having slow conversion rates to substances which no longer dissolve in the organic phase (such as NOX at low concentrations etc.) requires a high number of wash or extraction stages in countercurrent or else in crosscurrent or a combination thereof, associated with a disproportionately high extra consumption of wash medium and consequently extra occurrence of wastewater, which in turn leads to unacceptable capital and operating costs.
In addition, EP 0 897 907 B1 describes a process for recovering nitric acid into a mixture of aromatic compounds, wherein two supplementary process steps are to be used for purification of the crude nitration products: the crude nitration products, according to EP 0 897 907 B1, prior to the actual wash, are subjected to a distillation or a stripping operation for removal of the nitric acid, in order to remove the nitric acid dissolved or suspended in the crude nitration products prior to the wash in an at least partly separate manner and recycle it after appropriate treatment. However, this process, compared to the prior art outlined above, for purification of crude nitration products, requires considerable additional technical complexity, without being able to achieve an improved product purity compared to the prior art outlined above. As well as the waste stream from the downstream wash which is always still necessary (i.e. generally two waste streams from the wash, mainly wash acid from the acidic wash and wash alkali from the basic wash), the upstream step of distillation or stripping generates an additional third waste stream comprising dilute nitric acid, which has to be processed separately. If the waste stream from the wash and from the upstream distillation or stripping stage are combined for the purposes of common further processing, as well as the elevated technical complexity, there is no longer any advantage compared to a wash according to prior art. The upstream distillation or stripping, moreover, serves exclusively for nitric acid removal and recovery, but does not enable any improvement in relation to the removal of the impurities that are difficult to remove from the crude nitration products. Nor is there any focus on the peculiarities as occur specifically in the purification of crude dinitrotoluenes, but merely a quite general emphasis on the dinitration of any desired aromatic compounds. Moreover, the process proposed in EP 0 897 907 B1 constitute an elevated safety risk compared to the prior art and is therefore unsuitable especially for the purification of crude DNTs: given a content of up to 10% nitric acid in the crude nitration products and at temperatures up to 130° C. in the proposed distillation, oxidative and uncontrolled breakdown of the impurities present in the crude nitration products (for example nitrocresols, nitrobenzoic acids etc.), and also of DNT itself, cannot be ruled out with certainty. The process described therein is consequently unsuitable for the efficient and safe purification specifically of crude dinitrotoluenes. More particularly, it is not possible by the process described therein to remove trace impurities in order to produce high-purity DNT.